Abstract Married heterosexuals enjoy greater health and longevity than the unmarried, and the importance of marriage for health increases as people age. However, since same-sex couples cannot legally marry in most parts of the United States, we know very little about the health implications of marriage for aging gays and lesbians. The proposed project will provide the first in-depth and systematic analysis of legal marriage and health to compare, gay, lesbian, and heterosexual couples. The major goal of the proposed project is to assess how married men and women promote or deter one another's health, and how these processes vary across gay, lesbian, and heterosexual marriages. The primary focus of this project is on revealing the pathways and processes through which gay, lesbian, and heterosexual spouses influence each other's health, therefore we will collect new data and use an analytic approach that is uniquely suited to addressing issues of process. The R21 period will be used to construct a novel three-part mixed-methods dataset. We will conduct face-to-face 60- minute baseline surveys with open and closed-ended responses, and collect 14 days of daily process data from both spouses in 60 gay marriages, 60 lesbian marriages, and 60 heterosexual marriages (N=360 individuals, 180 couples, 5,040 diary days). Daily process (also known as diary) data are uniquely suited to investigating psychosocial processes that unfold in daily interactions with spouses, while also accounting for the contexts of those interactions. The diary data obtained from both spouses in each marriage will be analyzed in tandem to assess daily fluctuation in spousal dynamics and health outcomes (i.e., health behaviors, psychological state, and physical health symptoms) as well as mechanisms (e.g., stress, social support) that link spousal dynamics to health outcomes. Open-ended questions from the baseline survey and previously collected in-depth interview data will also provide qualitative data to analyze meanings and processes that explain and elucidate significant patterns revealed in the daily process analysis. This mixed methods project will inform health policy involving the gendered dynamics through which marriage influences health across couple type. We must identify areas of greatest risk and resilience for men and women in gay, lesbian, and heterosexual marriage to ground the most effective strategies for policy and practice. The purpose of the R21 mechanism is to support preliminary and novel projects that subsequently lead to larger-scale studies. We will utilize findings from the proposed project to ground the development of a future R01 application to create a national dataset on marriage and health in gay, lesbian, and heterosexual populations.